232 Part III. — Twenty-fifth Annual Report 



Infusors and Decaying Eggs. — On June 10 a big heap of eggs 

 remained near the mussel. A sample was examined. They were dis- 

 organised. There was a great quantity of very small and also big infusors 

 among them, and the smallest iufusor was not too big to get into the egg 

 by the micropyle. They did not appear to be able to get into the egg. 



Next day the infusors were quiet, apparently encrysted. On June 13, 

 in the same dish, the infusors were big and again active. The eggs have 

 almost altogether disappeared. I noticed infusors darting into the egg- 

 shells, going round inside and then out again. In one egg-shell two ^ 

 infusors were lying still. 



On June 15 the mussel was dead. 



Development. — According to Wilson — "In 20 hours or a shorter 

 time after fertilization, the ciliated embryo rise, it may be to the very 

 surface, in warm, calm, water. They are moved hither and thither by the 

 currents and by their own locomotor apparatus, the cilia, and latterly 

 the velum." 



Scott observed that the embryos rose "from the bottom at the end of 12 

 hours. They remain free-swimming for at least four days." 



Some spawn that was observed on June 30 at the Bay of Nigg was 

 examined. The eggs were in various stages of segmentation up to the 

 ciliated larva. The motion in the eggs and larvae was very slight, and 

 it is questionable if it would have been sufficient to raise them off the 

 bottom. Some of the eg-^s remained on the bottom next day. 



When 12 days old, the larval mussels are, Wilson says, almost semi- 

 circular, the hinge-line being straight. In later forms, the prismatic shell- 

 substance begins to be deposited round the margins of the valves. The 

 growth takes place most rapidly antero-posteriorly, the mussel thus as- 

 suming the elongated characteristic form of the adult. With the advent 

 of the prismatic shell, the velum is atrophied and the mussel sinks to the 

 bottom or lights on seaweeds, zoophytes, etc. The foot, at first vermiform 

 and highly extensile, is used for progression ; at the proximal region of the 

 foot the byssus-gland secretes the byssus wherewith the mussel may be 

 anchored. 



In July and August, M'Intosh observed that the surface of St. 

 Andrews Bay swarmed with minute mussels of a somewhat circular outline 

 and showing four branchial processes behind the foot. The young mus- 

 sels, according to the same author, are found settled on zoophytes in 

 July. At Kiel, Mobius states that the young mussels attach themselves 

 in the latter half of June. 



The Spawning of Modiolus. 



Female — February 27, 1907. — The eggs in a Modiolus examined were 

 nearly ripe, and they formed a brick-red dust to the naked eye. The 

 most of them showed a germinal vesicle in the middle of the opaque-red 

 yolk. Lots of the eggs are pitcher or pear-shaped. They measure "Imm. 

 in diameter. Some were ruptured. 



Male — June 9. — Two Modiolus that were kept in a tank measuring 

 5 feet by 4 feet by 2 feet deep, were seen to be spawning. A grey cloud 

 was seen to spread from them over the bottom of the tank and ascend in 

 a wide spiral up to the surface (fig. 98). The motion was due to the 

 current of water in the tank. The cloud of sperms formed a thick 

 layer on the surface of the water. A drop of water that was just 

 covered by a cover-glass |-in. in diameter contained 11 spermatozoa. The 

 sperm resembled much that of Mytilus edulis. Round the head there 

 appeared a lighter ring — a sort of halo. After some time the whole of the 



